Catheters are used in the field of medicine in a variety of medical and surgical procedures. For example, catheters are used extensively for delivering diagnostic or therapeutic agents to a selected site within the body. Microcatheters are used in neurointerventional and similar procedures. These catheters are commonly threaded through a vessel or artery and frequently follow a tortuous path in order to reach the site where the agent is to be applied. A balloon catheter for treating, for example, arterial stenosis has an inflatable balloon at its distal end. This catheter also follows a tortuous path to reach the site of the arterial restriction. The balloon is then inflated via a lumen through the shaft of the catheter, applying pressure to expand the stenosis against the artery wall.
Because these catheters often must be threaded through a tortuous path, the catheter must be rigid enough to allow the distal end of the catheter to be manipulated by the physician or surgeon. On the other hand, the catheter must be quite flexible to permit it to follow the tortuous path to the desired site of application. In order to satisfactorily meet the requirements of flexibility and also stiffness for manipulation, various designs of catheters are known and used. One such catheter utilizes a flexible catheter with an inflatable balloon at its distal end which, when partially inflated, will be carried by the blood flow to the desired location. Such catheters, however, cannot be used if the site where the agent is to be applied can be accessed only through a vessel that has a low blood flow rate.
More commonly, guide wires are used which can be advanced to the site, and with the guide wire in place, the catheter can then be telescoped over the wire and advanced to the application site. Catheters that use the guide wire technique, however, still must be sufficiently flexible to track the wire and sufficiently rigid so that the catheter can be advanced without buckling at the proximal end.
In order to overcome these limitations and difficulties, differential stiffness catheters have been developed which have different degrees of flexibility throughout their length. These catheters have a long and stiff proximal section coupled to a short and soft or flexible distal section that will track the guidewire. With these differential stiffness catheters, the physician or surgeon can push and maneuver the stiff proximal end to effectively advance the soft distal end. For example, Engelson U.S. Pat. No. 4,739,768 discloses a catheter which has a relatively stiff proximal segment and a relatively flexible distal segment, the segments being formed by forming the proximal segment of inner and outer coaxial tubes, one of which is relatively stiff, and the other of which is relatively flexible. The distal segment is then merely an extension of the relatively flexible tube.
Because this type of differential stiffness catheter is usually made by hand by joining two or more pieces of tubing together, they are labor intensive and therefore expensive to manufacture. Moreover, these catheters tend to buckle and to kink at the joints where there occurs an abrupt change in stiffness. Buckling and kinking are very undesirable characteristics for catheters. Also, there is a tendency for the joints to separate leaving the tip of the catheter inside the body and requiring surgery to retrieve it. Attempts have been made to reduce the buckling and kinking problems and prevent joint separation by making the catheter with a relatively soft layer throughout the entire length of the catheter, but this construction results in reduced stiffness at the proximal end.
Prior art patents such as U.S. Pat. Nos. 5,125,913 to Quackenbush and 4,250,072 and 4,283,447 to Flynn recognized some of the potential benefits of using a process technology called co-extrusion to make variable stiffness catheters. However, disappointing results have been obtained in following their teachings. Co-extruded catheters produced by periodic interruption using prior art teachings result in undesirably long transition sections, which are the sections of the catheter where the tubing changes from a stiff tube to a soft tube. Some of the catheters produced by these prior art processes have transition sections that extend the entire length of the catheter. These undesirably long transition sections have been the major problem in attempts to make catheters and other medical tubing with interrupted layers or interrupted elements. Also, the interrupted layers co-extrusion process results in only a moderate difference in stiffness between the proximal and distal sections--less than is considered desirable for catheters. Moreover, since very long cycle times are required for known interrupted layer processes, these co-extrusion processes are not as economically feasible as first thought. Further study has shown that these deficiencies cannot be corrected by simple means, such as process variable changes, but rather require fundamental changes in the process itself.
Furthermore, the prior art does not recognize the possibility of forming a very secure joint between soft and stiff resins by using co-extrusion and sequential extrusion processes to produce a "wedged-in" transition section in which one resin is securely locked or wedged into another resin.
In addition to the foregoing, medical catheters must have the proximal end attached to a variety of different connectors which facilitate attachment of one or more medical device or devices necessary to carry out the particular medical procedure using the catheter. At the point of attachment of the catheter tube to the connector, kinking can easily occur and restrict the flow of the fluid being introduced into the catheter tube. To minimize the probability of kinking, prior art catheters commonly use a short length of a flexible rubber tube that extends from the connector and into which the proximal end of the catheter is inserted and affixed. Although use of this rubber tube reduces kinking of the catheter tube, there is still considerable strain applied to the catheter tube at the point where it exits the rubber tube connector. This kinking problem also exists in tubing used in nonmedical applications.
It is therefore an object of the invention to develop a co-extrusion method of forming tubing such as catheters in which the transition section can be shortened and controlled to the point where a variety of suitable medical catheters with interrupted layers and elements can be properly produced at a reasonable cost.
It is another object of the invention to develop a method of forming the "wedged-in" construction in the transition section of the tubing to produce an extremely secure joint between different resins.
It is yet another object of the invention to use the methods of the invention to provide an improved strain relief section where the tubing is joined to a connector or other device thereby eliminating kinking at this joint.
It is a further object of the invention to develop a suitable co-extrusion head and system to carry out the methods of the invention.